It is known that dialdehydes are effective hardeners when used in photographic gelatino-silver halide emulsions. U.S. Pat. No. 3,232,764 discloses hardening photographic silver halide emulsions with dialdehydes whose aldehyde groups are separated by an unbranched chain of 2-3 carbon atoms, preferably glutaraldehyde and its derivatives. These compounds demonstrate excellent hardening properties, especially in view of the mechanical strength and temperature stability attained.
It is known, however, that aldehyde hardeners sometimes exhibit an undesirable influence on the sensitometric properties of photographic emulsions. Therefore, in actual practice, it is necessary to reach a compromise between sufficient hardening and the desired sensitometric properties. Generally, the process is carried out in such a way that the concentration of the hardener is as low as possible to maintain the desired mechanical strength and temperature stability of the films. For example, U.S. Pat. No. 3,232,764 states that to attain the desired hardening effect it is necessary to use the dialdehyde compounds in a concentration of at least 0.5 to 25% by weight of the gelatin content. A decrease of the concentration is not possible, because, as seen from Example 3 of the patent, with lesser amounts of hardener there is insufficient hardening for all practical purposes.
It is also known to add hardeners at any point in time during the preparation of the photographic gelatino-silver halide emulsions. There are also known processes and apparatus to accomplish this, especially shortly before coating, to prevent any premature secondary reactions of the hardener with any of the other emulsion ingredients.
It is the object of the invention to give an improved process for hardening photographic gelatino-silver halide emulsions using glutaraldehyde or substituted glutaraldehyde, whereby a combination of good hardening effects and improved sensitometric properties can be attained.